The present invention is in the field of control systems, and more particularly, digital feedback control systems.
There are many systems in the prior art which include a device-to-be-controlled embedded in a digital feedback loop, where that device may provide an output signal representative of its operation. Typically, the output signal is coupled by way of a feedback network to a difference network which provides an error signal to drive the device-to-be-controlled. The error signal is generally representative of the difference between an input command signal and the feedback signal provided by the feedback network. By way of example, where the device to be controlled is an induction generator, the output signal may be a relatively high frequency carrier voltage signal having a relatively slowly varying envelope. In an amplitude control loop configuration, the feedback network includes a sensor network responsive to the output signal which provides a feedback signal representative of that output signal. In various configurations, the error signal may be applied to the device-to-be-controlled by way of a compensation device or network which modifies the loop transmission characteristics in such a way to maintain stable loop operation. Various portions of the system may be preformed digitally with the use of suitable analog-to-digital (A/D) and digital-to-analog (D/A) conversion techniques. For example, a microprocessor network may be used to perform the difference network function.
In many cases, it is necessary to include a filter (usually lowpass in nature) in the feedback path to remove high frequency noise from the feedback signal. Various implementations of filters are known, varying from pure integrators, to multiple low, or bandpass filters. All of these networks are characterized by frequency responses with reduced gain at high frequencies relative to low frequencies. As a consequence, the output from the feedback network is generally a weighted average of past inputs. Thus, the feedback network may be considered to maintain a certain amount of memory. Because of this "memory", the feedback network generally introduces a phase lag to the control loop. This lag can de-stabilize a system, particularly during large signal operation. Furthermore, the response of the feedback network does not track the system output instantaneously, thereby degrading transient performance of the loop.
Moreover, in many configurations, the feedback network must be of relatively high quality in a precision loop in order to prevent limitations in loop accuracy due to drift. Prior art automatic correction techniques are known to correct for long-term drift. However, such techniques are relatively expensive, and depend on the feedback network maintaining substantially constant performance at least for times short compared with the loop memory. Moreover, as filtering is added to reduce drift, the loop memory increases, resulting in more critical specifications for the feedback network.
It is an object of the present invention to provide an improved digital feedback control system.
A further object is to provide an improved digital feedback control system which minimizes phase delay to the loop.
Yet another object is to provide an improved sensing system with substantially no memory, thereby permitting relaxed drift requirements and increased response speed.